Chapter 7

Chapter 7

Chapter 7LIGHT, COLOUR AND SIGHT.7.1 Formation of Images.

Formation of images by plane mirrors.

images formed by a plane mirror are caused by reflection of light from the mirror surface.

the characteristics of an image formed by a plane mirror are :

a) virtual

b) inversed laterally

c) upright

d) the same size as the object

e) image distance and object distance from the plane mirror are the same.

formation of image on a plane mirror

Formation of images by lenses.

the image formed by a convex lens depends on the distance of the object from the lens.

therefore, the characteristics of the image formed by a convex lens are

a) virtual or real

b) upright or inverted

c) diminished or enlarged

Convex lensConcave lens

Type of lensBiconvex lens

Biconcave lens

RoleConverges light rays that passes through it.Diverges light rays that passes through it.

Characteristics of imageFor a distant object, the image formed is real, inverted and smaller than the object.For a near object, the image formed is virtual, upright and bigger than the object.Image formed is virtual, upright, and diminished.

Ray diagrams. a ray diagram is a diagram which shows the path of light rays passing through a lens.

object distance is the distance between object and optical centre.

image distance is the distance between image and optical centre.

for a convex lens, light rays that are parallel with the principle axis will converge at the focal point behind the lens.

for a concave lens, light rays that are parallel to the principal axis will diverge after passing through the lens causing the focal point to be placed in front of the lens.

Construction of ray diagrams. there are specific procedures to follow when constructing ray diagrams.

when drawing ray diagrams, the following should be adhered to :

all rays from the object should be drawn with solid lines with the direction marked.

virtual rays should be drawn using broken lines.

real images should be drawn with solid lines and virtual images should be drawn using broken lines.

Ray diagrams of the formation of images by convex lens at different object distances.Object more than 2F

The image formed is :

real

inverted

smaller than the size of the object

Object at 2F


real

inverted

same size as the object

Object between F and 2F


real

inverted

larger than the size of the object

Object at F


image is formed at infinity

Object less than F


virtual

upright


7.2 Formation of Image by Optical Instruments.

Optical Instruments.

Periscopes

are normally used in submarines to look at objects on the water surface. a periscope consists of two plane mirrors arranged parallel to one another at an angle of 45o . the image formed by a periscope is virtual upright the same size as the object at the same distance from the object. the mirrors in the periscope can be replaced by a prism. the image formed will be clearer.

a periscope works based on the principle of the reflection of light

Magnifying glass

a magnifying glass is used to observe small objects. the image formed is larger than the size of the object.

is made of a biconvex lens.

when holding a magnifying glass, the object distance is less than the focal length thus, the image formed is

virtual

upright


a magnifying glass

a ray diagram showing the formation of image by a magnifying glass

Microscope

is used to observe tiny object.

consists of two convex lenses.

when observing a tiny object, the tiny object must be out in front of the objective lens.

the objective lens forms an image which is

real

inverted


the eyepiece acts as a magnifying glass to enlarge the image formed by the objective lens is

virtual

inverted

enlarged

Telescope

is used to look at distant objects.

also consists of two concave lenses.

the objective lens has a long focal length while the eyepiece has a short focal length. a distant object is focused by the objective lens and forms an image at the focal point is

real

inverted

smaller than the size of the object

the eyepiece acts as a magnifying glass to enlarge the real image formed by the objective lens is

virtual

inverted


Camera a camera consists of three main parts the lens, the shutter and the film.

the lens focuses the image of an object onto the film, the lens is adjusted by the focus adjuster either towards or away from the film to obtain a sharp image of an object.

to focus on a nearby object, the lens is adjusted away from the film to obtain a sharp image.

to focus on a distant object, the lens is adjusted towards the film to obtain a sharp image.

light rays that enter will pass through the diaphragm. the diaphragm changes the size of the aperture to control the amount of light entering the lens.

the image on the film is real, inverted, and smaller than the size of the object.

Part of cameraFunction

Lens (convex)Focuses the image of a distant object on the film.

ApertureAllows light to enter the camera.

DiaphragmTo control the camera aperture that allows light to enter the camera.

FilmActs as a screen to receive an object image that captured on it.

Comparison between eye and camera.

the eye of human functions like a camera. the eye lens focuses light onto the retina to form an image.comparison between eye and camera.

comparison of the structure of the eye and that of the camera to their functions.

Part of the eyeFunctionPart of the camera

Eye lensFocuses light to form an imageCamera lens

IrisControl the size of aperture and amount of light that enterDiaphragm

PupilAllows light to enterAperture

Ciliary bodyChanges the size of lens to obtain a sharp imageFocus adjuster

RetinaActs as a photosensitive layer to capture imageFilm

the characteristics of an image formed on the retina of the eye are real, inverted, and smaller than the size of the object. the ciliary body regulates the thickness of the eye lens. (this regulates the focal length of the eye lens)

when looking at a distant object, the ciliary body contracts and the lens becomes thinner.

when looking are a nearby object, the ciliary body relaxes and the lens becomes thicker.

the focus adjuster regulates the distance between the lens and the film to obtain a sharp image.

when taking a picture of a distant object, the focus adjuster is adjusted so that the lens is nearer to the film.

when taking a picture of a nearby object, the focus adjuster is adjusted so that the lens is further away from the film.

the size of the pupil and the amount of light entering the eye are controlled by the iris.

if we enter a dimly lit room, the size of the pupil enlarges and the amount of light entering the eye increases.

if we come out form a dark room to a brighter place and the amount of light entering the eye decreases. the size of aperture and the amount of light entering the camera is controlled by the diaphragm.

when taking a picture under a dark conditions, the diaphragm increases the size of the aperture and the amount of light entering the camera increases.

when taking a picture under bright conditions, the diaphragm decreases the size of the aperture and the amount of light entering the camera decreases.7.3 Light Dispersion.

is a process in which white light is split into its colour constituents called spectrum when it passes through a prism. a spectrum consists of seven colours in this order :

1. red

2. orange

3. yellow

4. green

5. blue

6. indigo

7. violet

light dispersion occurs because each colour constituent travels at different speed through the prism.

therefore, the coloured lights are refracted at different angles.

violet light is refracted more by a glass prism compared to red light which has a longer wavelength.

this is because violet light travels at the lowest speed.

Formation of rainbow. is natural phenomenon related to the dispersion of light. a rainbow can be seen during a drizzle in the morning or afternoon when sunlight shines on the raindrops. a raindrop acts as a prism.

a rainbow is formed when sunlight passes through raindrops.

the sunlight is retracted and dispersed into its colour constituents.

Formation of rainbow

(a) sunlight shines on the raindrop.

(b) a part of the sunlight is reflected off the outer surface of the raindrop.

(c) a part of the light is refracted at different angles in the raindrop.

(d) the light is split into its colour constituents through dispersion.

(e) the colour constituents are reflected off the far inner surface of the raindrop.

(f) light refraction occurs again as it exits the raindrop.

(g) a rainbow is formed in the sky.

7.4 Light Scattering. the earths atmosphere contains particles like gas molecules, vapour, and dust. when white light incidents these particles, light rays are obstructed and reflected.

these light rays scatter in all directions. (this occurrence is called light scattering)

light scattering is related to light colour; blue light is scattered more compared to red light because blue light has a shorter wavelength.

lights with shorter wavelength are refracted more.

examples of light scattering which occurs as natural phenomena are :

a) the blue sky during the day

b) the red sky during sunset

c) the red sun at sunset

blue light which a shorter wavelength is scattered more in all directions.

this causes the sky to appear blue.

at sunset, sunlight passes through a thicker atmospheric layer.

red and orange lights with long wavelengths pass through the atmosphere without disturbance because the other colours are scattered.

consequently, red and orange lights can be observed.

this causes the sun or the sky to appear red or orange at sunset.

7.5 Addition and Subtraction of coloured lights.

Addition of coloured lights.

primary colours are colours which cannot be obtained from mixing other colours. there are three primary colours, namely red, blue, and green. secondary colours are colours produced by adding primary colours. there are three secondary colours, namely yellow, magenta, and cyan. addition of primary colours and secondary colours and secondary colours can produce white light. for example, yellow light mixed with blue light will gave white light.

Subtraction of coloured lights.

coloured filters consist of primary and secondary filters. primary filters consist of red, green, and blue filters. primary filters only allow lights of the same colours to pass through them, other coloured lights are absorbed. ex. a green filter only allows green light to pass through it, other coloured lights are absorbed. secondary filters consist of yellow, magenta and cyan filters. secondary filters allow lights of the same colours and primary colours that form them to pass through them. ex. a yellow filter allows yellow light and primary colours that form it, namely red and green lights, to pass through it. the absorption of coloured lights by coloured filters is called subtraction of coloured lights.7.5 Principle of Subtraction of Coloured Lights to Explain The Appearance of coloured objects.

the colour of an opaque object depends on the colour of the light that it reflects.

an object of a primary colour, i.e. red, blue or green, only reflects light of the same colour.

for example, a blue object only reflects blue light.

an object of a secondary colour, i.e. yellow, magenta, or cyan, reflects light of the same colour and the primary colours that form it. for example, a magenta object reflects magenta, red, and blue lights.

a white object appears white in white light because a white object reflects all colours. (no coloured lights are absorbed)

a black object appears black because all coloured lights are absorbed by it. (no coloured light is reflected) absorption of light by coloured objects is based on the principle of subtraction of coloured lights.

the colour of an object depends on

a) the colour of the light the strikes it

b) the colours of the light absorbed

c) the colour of the light reflected

Function of rod cells and cone cells in the eye.

the retina contains two types of cell that are sensitive to light stimulus rod cells

is sensitive to light of low intensity

such as night

not sensitive to colour and only a black and white image is produced.

cone cells is sensitive to light of high intensity to detect colour there are three types of cone cell, and each is sensitive to red, green, and blue light respectively. other colours which can be seen are the products of mixing the three colours.Effects of mixing pigments,

1. The primary colours of pigments are red, blue, and yellow.2. Mixing red, blue, and yellow in different amounts gives various colours.

3. When these colours are mixed in the correct amounts, black is obtained.

4. Mixing pigments is based on the principle of subtraction of coloured lights.

5. Pigments are not as pure as coloured lights. Pigments have the property of absorbing and reflecting certain colours when shone on by white light.

6. For example,

a. blue pigment reflects violet, blue and green lights.

b. magenta pigment reflects red and blue lights.

7. When two pigments are mixed, the colour produced is the colour reflected by both pigments.

8. All colours, except while, can be produced by mixing coloured pigments.

Colour of pigmentsColour obtained

Red + YellowOrange

Cyan + YellowGreen

Magenta + YellowRed

Blue + YellowGreen

Magenta + Yellow + CyanBlack

colours obtained from the mixing some coloured pigments.

7.8 Importance of Colour in Daily Life.

Uses of colour in daily life.

The uses of colour in our daily life are as follows :

(a) printing (c) traffic lights

(b) electrical wiring (d) symbols and lights

Printing

colour pictures are printed by using four colours yellow, magenta, cyan, and black on separate plates.

the black colour is used to make the picture appears sharp and clearer.Stage 1 The image is printed with yellow followed by magenta.

Stage 2 The product of Stage 1 is printed with cyan.

Stage 3 Finally, the picture is printed with black to make the picture clear.

Colour printing

the international colour code is used in the wiring of three-pin plugs.

live wire is brownish or red in colour

neutral wire is blue

earth wire is yellow with green stripes or just green in colour

Traffic lights

a traffic light consists of red, yellow, and green lamps.

each colour represents a certain signal.

red stop

yellow (amber) ready to stop

green go

Symbols and signals

the red light of an ambulance indicates an emergency.

the yellow line by the roadside indicates a no-parking area.

the white line in the middle of the road separates the left and right lanes of the road.

the two white lines in the middle of the road indicate that no overtaking is permitted.

Importance of colour to living things.

colour is also important to plants and animals. colour enables animals to protect themselves form danger or to make it easy for them to hunt their preys by means of camouflaging to the surroundings. some animals have the ability to change the colour of their bodies according to the colour of the surroundings. the males of many species of birds, like the peacock, have colourful feathers to attract the attention of the females for mating purpose. colourful flowers attract the attention of insects for the pollination purpose. poisonous animals usually have bright colours to warn other animals. for example, toads that have bright colours are usually poisonous.object

image

observer

plane mirror

normal

Focus point

Optical centre

Focus point

Optical centre

Similarity

Addition of coloured lights

Mixing coloured pigments

Can be mixed to get other colours

Difference

Impure

Subtraction of colour

Red, blue, yellow

Red, blue, green

Addition of colour

Pure

Product of colour mixing

Primary colours

Method of colour mixing

Purity of colour

PAGE 2

Documents

Chapter 7